Conventionally, there has been known a crane such as a luffing jib crane which includes a jib on a distal end of a boom or a tower crane. In such a large-sized crane having the jib, the jib has the split structure formed of a plurality of parts (partial jibs). In a site where the crane is used, the jib is assembled by connecting partial jibs to each other.
In general, the large-sized crane is assembled in a state where the crane is laid down on a ground such that the boom and the jib extend on one straight line (so called “lying-type assembly method”). However, in such a lying-type assembly method, it is necessary to ensure a space elongated in a horizontal direction so that the boom and the jib can be extended horizontally. However, when the large-sized crane is used, there is a possibility that it is impossible to ensure a sufficient space which allows the adoption of the lying-type assembly method in a work site.
In the case where a sufficient space cannot be ensured so that the lying-type assembly method cannot be adopted in assembling a crane, a method referred to as a “standing-type assembly method” is adopted where a jib is mounted on a boom in a state that the boom is raised. For example, JP-A-2008-81242 describes a crane assembling method where partial jibs are sequentially connected one by one to a distal end of a boom to assemble a crane.
In the crane assembling method of a crane described in the above-mentioned publication, firstly, the boom is mounted on a crane body, and a base end jib (a partial jib connected to the boom) is connected to a distal end of the boom laid down on a ground. Then, an upper side portion of a distal end of the base end jib and an upper side portion of a proximal end of an intermediate jib (a partial jib other than both ends) placed on the ground are joined to each other by a pin so that a temporary connection state is brought about where the intermediate jib is bent toward a back side (upper side) with respect to the base end jib. Next, ropes are extended between a strut supported on the boom above the base end jib and both ends of the intermediate jib. By raising the boom in a state where the intermediate jib is held horizontally, the boom is lifted in air while maintaining the temporary connection state where the base end jib and the intermediate jib are bent. Then, by slackening the above-mentioned ropes which hold the intermediate jib, the intermediate jib rotates relative to the base end jib due to its own weight. Then, by joining a lower side portion of a distal end of the base end jib and a lower side portion of a proximal end of the intermediate jib to each other by a pin, the intermediate jib is connected to the base end jib such that the intermediate jib extends straight with respect to the base end jib. At this stage of operation, to prevent the base end jib from excessively sagging downward more than a fixed angle with respect to the distal end of the boom, the crane described in the publication includes a back stop cylinder on a distal end portion of the boom. The back stop cylinder connects the distal end portion of the boom and the base end jib to each other thus preventing lowering of the base end jib.
The crane assembling method described in the above-mentioned publication is applicable to only a special crane which includes the back stop cylinder for preventing sagging of the base end jib.
Further, in the case of assembling a large-sized crane having a jib in a narrow space, a method is also considered where an upper side portion of a distal end of a base end jib and an upper side portion of a proximal end of an intermediate jib placed on a ground are joined to each other by a pin, both ends of the intermediate jib are separately lifted, and the intermediate jib is connected to the base end jib in a straight posture. However, such a method requires a sophisticated crane operating technique, and requires a particular attention for ensuring safety.